Dual stimuli-responsive supramolecular pseudo-polyrotaxane hydrogels

Article abstract of DOI:10.1039/c3sm27776c

THPP-(PEG₂₀₀₀-BA)₄, a four-branched molecule end-decorated with benzaldehyde (BA), was successfully designed and synthesized. It can form physical pseudo-polyrotaxane (PPR) hydrogels in the presence of α-cyclodextrins (α-CDs). The branched structure of the THPP core promotes gel formation with a small amount of host and guest. Moreover, these resulting hydrogels are dual stimuli-responsive, which can be observed by physically macroscopical phenomena and ¹H NMR spectra. Since BA can react with amine to form a pH-responsive Schiff-base which possesses a dynamic covalent nature, it is anticipated that the formation of gels can be modulated by pH. When THPP-(PEG₂₀₀₀-BA)₄ was blocked by the relatively large molecule 6-N-ethylenediamine-6-deoxy functionalized β-CD (EDA-β-CD) gelation did not take place. However, the addition of an acid resulted in gelation since the benzoic imine bonds can hydrolyze under acidic conditions and α-CDs can thus pass through BA to thread on the polyethylene glycol (PEG) chains. When a strongly competitive guest, 1-[p-(phenylazo)benzyl]-bromide (Azo-C1-N⁺), was added to the gel, a gel-to-sol transition was observed due to the disassembly of inclusion complexes between α-CDs and PEG chains. Simultaneously, this gel shows a photo-responsive capacity because of the presence of azobenzene. Therefore, the cycles of gel-sol transitions were achieved through pH- and photo-stimuli. This kind of hydrogel is promising for use in many fields, such as biology and electronics.

Full text of DOI:10.1039/c3sm27776c

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Dual stimuli-responsive supramolecular pseudo-
polyrotaxane hydrogels†
Cite this: DOI: 10.1039/c3sm27776c
Lipeng Zhou, Jiaxi Li, Quan Luo, Junyan Zhu, Huixin Zou, Yuzhou Gao, Liang Wang,
*
*
Jiayun Xu, Zeyuan Dong and Junqiu Liu
THPP-(PEG2000-BA)4, a four-branched molecule end-decorated with benzaldehyde (BA), was successfully
designed and synthesized. It can form physical pseudo-polyrotaxane (PPR) hydrogels in the presence of
a-cyclodextrins (a-CDs). The branched structure of the THPP core promotes gel formation with a small
amount of host and guest. Moreover, these resulting hydrogels are dual stimuli-responsive, which can
be observed by physically macroscopical phenomena and 1H NMR spectra. Since BA can react with
amine to form a pH-responsive Schiff-base which possesses a dynamic covalent nature, it is anticipated
that the formation of gels can be modulated by pH. When THPP-(PEG2000-BA)4 was blocked by the
relatively large molecule 6-N-ethylenediamine-6-deoxy functionalized b-CD (EDA-b-CD) gelation did not
take place. However, the addition of an acid resulted in gelation since the benzoic imine bonds can
hydrolyze under acidic conditions and a-CDs can thus pass through BA to thread on the polyethylene
glycol (PEG) chains. When a strongly competitive guest, 1-[p-(phenylazo)benzyl]-bromide (Azo-C1-N+),
was added to the gel, a gel-to-sol transition was observed due to the disassembly of inclusion
complexes between a-CDs and PEG chains. Simultaneously, this gel shows a photo-responsive capacity
because of the presence of azobenzene. Therefore, the cycles of gel–sol transitions were achieved
through pH- and photo-stimuli. This kind of hydrogel is promising for use in many fields, such as biology
and electronics.
Received 3rd December 2012
Accepted 27th February 2013
DOI: 10.1039/c3sm27776c
www.rsc.org/softmatter
interactions11 and dynamic covalent chemistry.12 Stimuli-
responsive materials formed by supramolecular interactions,
Introduction
Stimuli-response is a very widespread phenomenon in nature.
Learning from nature, many articial smart materials have been
designed and investigated to simulate this stimuli-responsive
phenomenon over the last few decades. These smart materials
have drawn great and growing attention from scientists world-
wide due to their interesting and charming properties which
can be remarkably varied in diverse fashions by external or
internal stimuli, including pH,1 temperature,2 redox,3 enzymes,4
light,5 magnetic elds,6 electric elds,7 strain,8 chemicals,9 and
so on. Meanwhile, smart materials with multiple stimuli-
responsive functions are particularly attractive. Generally, more
stimuli responses would generate more exibility to further the
applications of these materials. Thus, it is important to design
and achieve multi-responsiveness within a single so material,
not only for scientic research but also for potential
applications.10
such as host–guest chemistry, can be reversibly regulated by the
breaking and formation of noncovalent interactions, thus
resulting in the transformations of structures, morphologies
and even functions. Recently, dynamic covalent chemistry has
been a powerful tool for the preparation of stimuli-responsive
materials.13 Compared to noncovalent bonds, dynamic covalent
bonds are more stable due to the strong nature of the covalent
bonds.14 The use of dynamic covalent bonds to develop stimuli-
responsive materials will undoubtedly expand the applications
of such smart materials.
Hydrogels are three-dimensional polymeric networks,
which are hydrophilic and cross-linked via covalent bonds
or noncovalent interactions. There is great interest in the
biomedical applications of hydrogels because they are
homogenous so materials with physical properties similar to
so tissues.15 These materials have shown charming prospects
in various areas, including sensors, separation systems, drug-
carriers and biomaterials.16 Recently, considerable efforts
have been devoted to the construction of stimuli-responsive
hydrogels due to their extraordinary capabilities as “smart” so
materials that can be signicantly changed via volume phase
transitions or sol–gel transitions in response to an external
stimulus. However, most of the reported stimuli-responsive
Many stimuli-responsive materials have been designed
and prepared based on the concepts of supramolecular
State Key Laboratory of Supramolecular Structure and Materials, College of
Chemistry, Jilin University, Changchun 130012, People's Republic of China. E-mail:
zdong@jlu.edu.cn; junqiuliu@jlu.edu.cn; Fax: +86-431-85193421
† Electronic supplementary information (ESI) available: Experimental details and
Fig. S1–S9. See DOI: 10.1039/c3sm27776c
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hydrogels were designed mainly based on either noncovalent the reversible cycles of sol-to-gel and gel-to-sol transitions were
interactions or dynamic covalent bonds and hydrogels observed.
produced by combining both of them are very rare. Several
years ago, Harada et al. reported the self-assembly of PEG and
Experimental section
Materials
a-CDs to form linear PPR with the PEG axis threading a-CDs,17a
as well as the formation of physical hydrogels by PPR.17b,c
N,N-Dimethylformamide (DMF) was dried over calcium hydride
and distilled under reduced pressure. EDA-b-CD was prepared
as described previously.22 Nitrosobenzene, p-aminotoluene,
N-bromosuccinimide (NBS), benzoyl peroxide (BPO), poly-
ethylene glycol (PEG) (Mn ¼ 2000), 4-toluene sulfonyl chloride
(TsCl), 4-hydroxybenzaldehyde, a-CD, b-CD, pyrrole, K2CO3,
pyridine, acetic acid, propionic acid, triethylamine (TEA), eth-
ylenediamine (EDA), tetrahydrofuran (THF), acetone, CCl4,
methanol, ethyl ether and other reagents were commercially
available and used directly as received.
Currently there are many studies focused on these hydrogels in
virtue of their good biocompatibility and very promising uses
as biomedical materials. The formation mechanism and
structure of such PPR hydrogels have been well studied.18 At
the same time, Jiang and coworkers have investigated the
dissociation and reassembly of the PPR hydrogel by breaking
and forming the reversible host–guest interactions via the
alternation of visible and UV irradiation.19 They carried out a
systematic research and proposed another medium
(i.e., optical-stimulus) to regulate the PPR hydrogel besides
thermal and mechanical stimuli. Nevertheless, these PPR
hydrogels were formed only by reversible noncovalent bonds
and were single stimulus-responsive. To our knowledge, PPR
hydrogels designed by employing a combination of dynamic
covalent bonds and supramolecular interactions have not been
reported yet.
Taking advantage of supramolecular interactions and
dynamic covalent bonds, especially the benzoic imine bond20
which is stable under physiological conditions but can hydro-
lyze under certain extracellular pH conditions,21 we successfully
prepared a type of dual stimuli-responsive hydrogel with the
four-branched molecule THPP-(PEG2000-BA)4 (Scheme 1). THPP-
(PEG2000-BA)4 decorated with BA at the terminal end can react
with cyclodextrin amine so as to be capped under alkaline
Measurements
1H NMR spectra were recorded on a Bruker AVANCEIII500.
UV-vis spectra were taken on a Shimadzu UV-2450 spectropho-
tometer. Dynamic light scattering (DLS) experiments were
carried out on a Zetasizer Nano ZS with the scattering angle of
90ꢀ. High performance liquid chromatography (HPLC) experi-
ments were performed on an Agilent ZORBAX GF-250. Deion-
ized water was used as the mobile phase at the rate of 0.5 mL
minꢁ1. All the measurements were carried out at ambient
temperature.
Sample synthesis
conditions. As expected, a-CDs cannot be threaded on the PEG Azo-C1-N+ was synthesized according to the method reported by
linkers. By adjusting the solution to be acidic, the benzoic imine Jiang and coworkers,19a as shown in Scheme S1 in the ESI.†
bond hydrolyzes into benzaldehyde and ammonium. Subse- 4-Methyl azobenzene was prepared by stirring nitrosobenzene
quently, a-CD passes through benzaldehyde to form a PEG–a- and p-aminotoluene in acetic acid for 24 h. The intermediate
CD inclusion complex, and the solution converts into a gel. 4-methyl azobenzene was bromized with NBS to afford 4-bro-
Aer the addition of a photoresponsive azobenzene moiety to momethyl azobenzene. 4-Bromomethyl azobenzene was reacted
the PPR hydrogel, the hydrogel converts into a sol solution once with pyridine at reuxing temperature to afford the target
again. This resulting solution transforms back into a gel under molecule. The synthetic route of THPP-(PEG2000-BA)4 is shown
UV irradiation. By changing the pH and UV-visible irradiation in Scheme S2 in the ESI.†
Tetrakis-4-hydroxyphenylporphyrin (THPP) was synthesized
according to a previous method.23 Under a nitrogen atmosphere
the freshly distilled pyrrole (3.46 mL, 50 mmol) in 30 mL pro-
pionic acid was slowly added to 4-hydroxybenzaldehyde (6.11 g,
50 mmol) in the reuxing propionic acid (300 mL) solution.
Aer vigorous stirring for 3 h, approximately 200 mL propionic
acid were concentrated by vacuum distillation, and then 100 mL
ethanol were added aer the solution was cooled to room
ꢀ
temperature. Aer the mixture solution was kept at 4 C over-
night, the purple crystalline precipitate was ltered and puried
by column chromatography.
PEG2000 (20 g, 10 mmol), TsCl (1.72 g, 9 mmol) and TEA
(2.5 mL, 18 mmol) were dissolved in 200 mL THF in a round-
bottom ask. The mixture solution was stirred for 60 h at room
temperature and then ltered. The ltrate was concentrated,
and puried by column chromatography to afford PEG2000-OTs.
A solution of compound PEG2000-OTs (15 g, 7 mmol), 4-
Scheme 1 Chemical structures of Azo-C1-N+ and THPP-(PEG2000-BA)4.
hydroxybenzaldehyde (1 g, 8.2 mmol) and K2CO3 (4 g, 29 mmol)
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in 100 mL acetone was reuxed for 24 h. The precipitate was
ltered off. Aer the ltrate was evaporated under vacuum, the
residues were puried by column chromatography to afford
PEG2000-BA.
TsO-PEG2000-BA was prepared according to the synthetic
method of PEG2000-OTs. PEG2000-BA (10.5 g, 5 mmol), TsCl
(1.4 g, 7.3 mmol) and TEA (2 mL, 14 mmol) were dissolved in
200 mL THF in a round-bottom ask. The mixture was stirred
for 60 h at room temperature. The white precipitate was ltered
off under reduced pressure and the ltrate was concentrated.
The residues were puried by column chromatography to afford
TsO-PEG2000-BA.
A solution of TsO-PEG2000-BA (2.26 g, 1 mmol), THPP
(0.135 g, 0.2 mmol) and K2CO3 (1 g, 7.2 mmol) in 50 mL DMF
was stirred for 48 h at 80 ꢀC under an atmosphere of nitrogen.
The precipitate was ltered off and the solvent was evapo-
rated under vacuum. The residue was puried by column
chromatography to afford the target molecule THPP-
(PEG2000-BA)4. 1H NMR characterization is shown in Fig. S3 of
the ESI.†
Scheme 2 Schematic representation of the photoresponsive PPR hydrogels.
pseudo-polyrotaxane by their inclusion complexation. a-CD
becomes hydrophobic due to the consumption of hydroxyl
groups via the hydrogen bond interaction. The PEG–a-CD
complex domains aggregate by hydrophobic interactions.
The PEG chain is too long to be fully included by a-CD.
However, inclusion complex domains are long enough to
aggregate with each other. The aggregation results in the
formation of microcrystals which play an important role in
physical cross-links. As observed, the aqueous solution of
THPP-(PEG2000-BA)4 and a-CD can give rise to the generation
of a gel.
Results and discussion
The gelation ability of THPP-(PEG2000-BA)4
The gelation ability of the THPP-(PEG2000-BA)4 molecule
was investigated. Aqueous a-CD was mixed with THPP-
(PEG2000-BA)4 solutions at different concentrations followed
by sonication. We found that THPP-(PEG2000-BA)4 gelled with
100 mg mLꢁ1 a-CD at the concentration of 20 mg mLꢁ1 in 2 h
(Fig. 1A, Table S1 and Fig. S4 in the ESI†). As with the previous
report,17b we propose the gelation mechanism of THPP-
(PEG2000-BA)4 and a-CD (Scheme 2). a-CD can complex with
the exterior benzaldehyde by host–guest interaction at the
rst stage. Because of the reversibility of the supramolecular
interactions, a portion of a-CD moves from the benzaldehyde
onto the PEG chain. The uncomplexed benzaldehyde can
capture free a-CD in solution. Through such an exchange
process, a-CD can be threaded on the PEG chain across the
benzaldehyde continuously until almost no free a-CD exists
in solution. As previously known, PEG and a-CD can form
The 1H NMR study, as shown in Fig. 1C (also Fig. S7 in the
ESI†), proved our proposed mechanism. We found that when
the mixture solution retained a sol form, the chemical shi of
H5 of a-CD moves upeld (from 3.99 ppm to 3.95 ppm) and all
the proton signals of a-CD become broader 20 min aer mix-
ing THPP-(PEG2000-BA)4 and a-CD. When the mixture solution
transformed into a gel form, all the proton signals of a-CD
became wide 2 h aer mixing THPP-(PEG2000-BA)4 with a-CD.
The proton signal changes of a-CD in the 1H NMR spectra were
caused by the fact that the PEG chains threaded into the a-CD
cavities, suggesting the formation of a THPP-(PEG2000-BA)4–a-
CD inclusion complex.19a Additionally, due to the inclusion
complexation between benzaldehyde and a-CD, the proton
signals of benzaldehyde shied downeld (from 9.77 ppm,
7.90 ppm, and 7.76 ppm to 9.89 ppm, 8.14 ppm and 7.92 ppm,
respectively), as shown in Fig. 1B. The benzaldehyde moiety
could easily capture free a-CD compared to the PEG chain.
This was favorable for gelation and could shorten gelation
time. Dynamic light scattering (DLS) experiments (Fig. S6 in
the ESI†) were performed in a diluted solution in order to
monitor the formation of linear PPR. No aggregates were
detectable in the solution of THPP-(PEG2000-BA)4. Aer the
addition of a-CD, a broad peak, corresponding to a size
ranging from 70 to 600 nm, was observed. This peak was
attributed to the formation of the PPR aggregate consisting of
THPP-(PEG2000-BA)4 and a-CD.
Fig. 1 (A) THPP-(PEG2000-BA)4–a-CD hydrogel, THPP-(PEG2000-BA)4 20 mg mLꢁ1
and a-CD 100 mg mLꢁ1; (B) and (C) local 1H NMR spectra of (a) THPP-(PEG2000-BA)4,
(b) free a-CD, (c) THPP-(PEG2000-BA)4–a-CD sol (20 min after mixing) and (d) THPP-
(PEG2000-BA)4–a-CD gel (2 h after mixing) in D2O.
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Considering the similar chain length between PEG8000
and THPP-(PEG2000-BA)4, we select PEG8000 as a reference
compound. The concentration of a-CD was xed at 50 mg
mLꢁ1and different amounts of PEG8000 were mixed with a-CD
(Fig. S5 in the ESI†). The solution of PEG8000 and a-CD turned
turbid when the concentration of PEG8000 reached 10 mg mLꢁ1
,
but we did not observe gelation in 20 h. The minimum gel
concentration of PEG8000 is about 20 mg mLꢁ1. The gel
concentration of the guest (THPP-(PEG2000-BA)4) and host (a-
CD) (6.0 wt%) is lower than the previously reported results.19a
Obviously, the four-branched THPP core is helpful for gelation.
The branched center provides cross-linking regions benecial
to the formation of gel at low concentrations.
Photoresponsive ability of the hydrogel
The reversible sol–gel transition in the PPR hydrogel systems
consisting of PEG and a-CD has been reported by using a
competitive photoresponsive guest, such as Azo-C1-N+ due to
the conversion of the trans- and cis-conformations of Azo-C1-N+
by changing UV and visible light irradiations.19 As previously
known, a-CD has a higher association constant (ca. 104 Mꢁ1) for
trans-Azo-C1-N+ than for PEG. Keeping this in mind, we
successfully introduced the competitively photoresponsive
guest Azo-C1-N+ into our hydrogels, as shown in Scheme 2.
The equivalent molar amount of trans-Azo-C1-N+ relative to
a-CD was then added to the hydrogel. Aer the mixtures were
kept overnight at room temperature, the gel turned into a sol
(Fig. 2A). The aldehyde signals of benzaldehyde (Fig. 2E and S9
in the ESI†) shied back to less than 9.80 ppm, which indicates
that the benzaldehyde moiety moved away from the a-CD cavity.
The proton signals of Azo-C1-N+ shied downeld (for instance,
the methylene proton signals shied from 5.92 ppm (Fig. 2E) to
6.00 ppm and the aromatic proton signals shied from 8.97
ppm to 9.01 ppm) and the proton signals of a-CD moved upeld
(H1 from 5.07 ppm (Fig. 2F) to 4.97 ppm and H5 from 3.95 ppm
to 3.74 ppm). All the peaks of the proton signals of a-CD turned
back into the narrow ones, which was consistent with when Azo-
C1-N+ entered into the a-CD cavity, as shown in Fig. 2F-d. These
results demonstrated that trans-Azo-C1-N+ kicked the PEG chain
out of the a-CD cavity and instead formed the inclusion
complex with a-CD.
The subsequent UV irradiation at 365 nm of the resultant sol
for 1.5 h made it return to a hydrogel (Fig. 2B). As seen in the UV-
vis absorption spectra (Fig. 2D and S8 in the ESI†) the absorption
band at 324 nm, corresponding to p–p* transition of the trans-
form of the azo group, evidently decreased, and hypsochromi-
cally shied to 294 nm aer UV irradiation. Meanwhile, the
absorption band at 426 nm, corresponding to n–p* transition of
the cis-form of the azo group, slightly increased. These variations
clearly showed the isomerization of Azo-C1-N+ from the trans-
form to the cis-form. As previously known, the cis-Azo-C1-N+
could not bind to a-CD because of the very weak affinity between
them. The chemical shis of cis-Azo-C1-N+ in Fig. 2E-c and e
underwent no change with or without a-CD. Aer UV irradiation,
Fig. 2 (A) THPP-(PEG2000-BA)4–a-CD–Azo-C1-N+ sol; (B) sol in (A) after UV irra-
diation; (C) gel in (B) after visible-light irradiation; (D) THPP-(PEG2000-BA)4–Azo-
C1-N+–a-CD before UV irradiation (black), after UV irradiation (red) and later after
visible irradiation (magenta); (E) and (F) local 1H NMR spectra of (a) THPP-
(PEG2000-BA)4; (b) and (c) Azo-C1-N+; (d) and (e) Azo-C1-N+–a-CD; (f) and (g)
THPP-(PEG2000-BA)4–Azo-C1-N+–a-CD in D2O. Thereinto, (b), (d) and (f) represent
the system before UV irradiation and (c), (e) and (g) represent it after UV
irradiation.
a-CD cavity and regenerated the PPR hydrogel. The 1H NMR
spectra proved these change processes. The methylene and
aromatic proton signals of Azo-C1-N+ moved upeld (5.77 ppm
and 8.85 ppm, respectively), which is in agreement with cis-Azo-
C1-N+. At the same time, the aldehyde signal of benzaldehyde
shied to higher values than 9.80 ppm, and the H1 signal of
a-CD moved to higher values than 5.00 ppm. The H5 signal of
a-CD remarkably shied downeld compared to before UV
irradiation. These changes were caused by the fact that the PEG
chain entered the a-CD cavity. The hydrogel returned to a sol
again as it was irradiated by visible light (Fig. 2C) due to the
regeneration of the trans-form (Scheme 2 and Fig. 2D). Such gel-
to-sol and sol-to-gel transitions were realized repeatedly as the
visible light and UV irradiation was alternately used.
The pH-controlled preparation of the hydrogel
the trans-Azo-C1-N+ became the cis-Azo-C1-N+, away from the The PEG-linker was blocked by the reaction of the THPP-
a-CD cavity. Simultaneously, the PEG chain entered into the (PEG2000-BA)4 molecule with EDA-b-CD in an aqueous
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Scheme 3 Schematic representation of the pH-controlled preparation proce-
dure of the PPR hydrogels.
dissociated. Therefore, the gel-to-sol transition cannot be real-
ized only by pH adjustment.
Fig. 3 (A) THPP-(PEG2000-BA)4–EDA-b-CD–a-CD sol under alkaline conditions; (B)
sol in (A) after acidification; (C) local 1H NMR spectra of (a) THPP-(PEG2000-BA)4; (b)
and (c) THPP-(PEG2000-BA)4–EDA-b-CD; (d) and (e) THPP-(PEG2000-BA)4–EDA-b-
CD–a-CD in D2O. Thereinto, (b) and (d) represent the alkaline conditions and (c)
and (e) represent the acidic conditions.
The dual stimuli-responsive PPR hydrogel
The hydrogel with the four components, THPP-(PEG2000-BA)4,
EDA-b-CD, a-CD and Azo-C1-N+, was pH- and photo-sensitive.
The sol solution remained under both acidic and alkaline
conditions (Fig. 4A and B). However, the sol solution became a
gel under acidic conditions with the assistance of UV irradia-
tion (Fig. 4D). Under alkaline conditions the sol solution
underwent no change upon UV irradiation (Fig. 4C). The
schematic representation of this process is illustrated in
Scheme 4.
solution. To make sure the PEG-linker was capped, triethyl-
amine (30 mL) was added to the solution of THPP-(PEG2000
-
BA)4 within a total volume of 1 mL. In the 1H NMR spectra
(Fig. 3C-b and S10 in the ESI†) we found that the peak at 9.77
ppm due to the proton on the benzaldehyde decreased and a
new peak at 8.44 ppm corresponding to the protons of the
benzoic imine conjugate structure appeared. These data
conrmed that a large proportion of the aldehyde groups were
converted into imines. Gelation did not take place aer a-CD
was added (Fig. 3A) and the phenyl signals of benzaldehyde
remained upeld. These results demonstrated that the benz-
aldehyde was not bound in the cavity of a-CD because the a-CD
cavity was too small to pass through the large b-CD. However,
with the addition of hydrochloric acid (20 mL) the acidic
solution quickly transformed into a gel (Fig. 3B). Since the
benzoic imine bond hydrolyzed under acidic conditions, a-CD
could pass through the smaller benzaldehyde to form a gel.
This observation was consistent with the 1H NMR results. The
proton signal of the benzoic imine at 8.44 ppm disappeared
whereas the proton signal of benzaldehyde at 9.81 ppm was
enhanced. In addition, the phenyl signal of benzaldehyde
shied downeld in the presence of a-CD. At the same time,
all the proton signals of a-CD shied upeld and became
broader. These experiments conrmed the formation of
the THPP-(PEG2000-BA)4–a-CD inclusion complex. This pH-
controlled procedure is illustrated in Scheme 3.
Fig. 4 The THPP-(PEG2000-BA)4–EDA-b-CD–a-CD–Azo-C1-N+ quaternary system:
This pH-stimulus procedure is irreversible. Aer the gel has
(A) and (B) after visible-light irradiation; (C) and (D) after UV irradiation; (A) and
adjusted to being alkaline, the PEG–a-CD complex can be hardly (D) under acidic conditions; (B) and (C) under alkaline conditions.
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Scheme
4
Schematic representation of the dual stimuli-responsive PPR
hydrogel.
Conclusions
In conclusion, we successfully designed and prepared a type of
supramolecular PPR hydrogel. This physical gel was dual
stimuli-responsive. UV-vis and NMR spectra were recorded to
monitor the stimuli-responsive procedure. The pH-adjustment
could control the gelation process by breaking and forming
dynamic covalent bonds. In the case of THPP-(PEG2000-BA)4
capped with a relatively large molecule, EDA-b-CD, gelation did
not take place. The addition of an acid caused gelation because
the benzoic imine bond hydrolyzed under the acidic condition
and a-CDs could thus pass through BA to be threaded on the
PEG chains. UV and visible irradiation could control the gel–sol
transformations by manipulating the reversible noncovalent
bonds between host a-CDs and the strongly competitive guest
Azo-C1-N+. Reversible cycles of sol-to-gel and gel-to-sol transi-
tions were achieved by means of the synergy of pH-adjustment
and UV-visible irradiation. We realized the control of the
dynamic covalent bonds and supramolecular interactions in a
single hydrogel system. The use of the THPP core could
signicantly decrease the gelation concentration. Supramolec-
ular PPR hydrogels containing the THPP core are promising
systems for many future applications, such as biomimetic
chemistry and electronics.
Acknowledgements
Financial support from the National Natural Science Foundation
of China (no. 21234004, 21274051,91027023, 20921003, 21004028)
and the 111 project (B06009) are greatly acknowledged.
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